In the United States there are about 550 tar sand occurrences known to exist in 22 states. These deposits are estimated to contain up to 50 billion barrels of crude. By far the most important deposits are the near-surface tar sands of Utah, which are estimated to contain 22-29 billion barrels of petroleum with 96% of the oil occurring in sandstone rock in six major deposits. Of the 50 billion barrels of the identified U.S. tar sand reserves, about 10% (5 billion barrels) are located close enough to the surface to be mined by conventional open pit techniques and extracted at the mine site.
A variety of techniques have been proposed for the surface extraction of bitumen from tar sands; e.g., hot or cold water with flotation, water-solvent mixtures, and solvent extraction. Of these techniques, water separation has an advantage in that the equipment requirements are relatively simple and it is an established, commercial method for the processing of Canadian tar sands. However, the water wet nature and bitumen composition of the Athabasca tar sands is unique; and, consequently, it has not been possible to directly apply the Canadian technology to U.S. tar sands.
Water separation processes are essentially mechanical methods. They suffer from the disadvantage in both the low efficiency of displacement of the bitumen from the sand and the poor flotation behavior of the released bitumen which latter is strongly influenced by changes in bitumen viscosity within a particular ore body. Water processes also require significant volumes of water which must be recycled to approach economical operation. Consequently, methods for minimizing the formation of oil/water emulsions and means of treating fine-clay/water suspensions are generally required. Efficient water recycle is not only important in order to avoid costly environmental problems, but it is also scarce and generally closely regulated in those areas of the U.S. where most tar sand deposits are located.
Water-solvent processes are chemical dissolution methods which offer the potential advantage of diminishing the energy associated with a sand drying operation, but suffer many of the disadvantages of the water extraction process with regards to clarifying and recylcing large volumes of water. Difficult to break water/oil/solvent emulsions also present a significant problem.
Solvent extraction appears to be especially suited for the surface extraction of the oil-wet tar sands found in the U.S. However, essentially all the developmental work which has previously taken place for the solvent extraction of tar sands has been carried out using hexane and similar light petroleum hydrocarbon solvents. These types of chemical extractants are not good solvents for bitumen. The asphaltenic content of bitumen (normally in the range of 15-25%) is not readily soluble in aliphatic hydrocarbon solvents. Consequently, slow dissolution rates, poor extraction efficiencies, column plugging due to reprecipitated asphaltenes, and the expense and difficulty required in recycling large volumes of such extremely hazardous solvents has discouraged many workers from pursuing a solvent extraction process approach for the recovery of bitumen from tar sands. The properties of several commercially-important chlorinated solvents could obviously overcome many of the objections inherent in the use of hydrocarbon solvents; however, they are generally perceived as not suitable for this application because of both their thermal and hydrolytic instability at elevated temperatures and consequent corrosion potential.
There are myriads patents which disclose processes for recovering bitumen from tar sands and oil-shale as well as unique and conventional solvent systems for use in particular processes having modified steps both in treatment and solvent recovery. Exemplary of these patents are Hastings, U.S. Pat. No. 4,311,561; L. I. Hart et al., U.S. Pat. Nos. 4,054,506 and 4,054,505; R. G. Murray et al., U.S. Pat. Nos. 4,120,775 and 4,176,465; T. A. Pittman, et al., U.S. Pat. Nos. 3,856,474 and 4,029,568; G. B. Karnofsky, U.S. Pat. No. 4,239,617; C. D. Smith et al., U.S. Pat. No. 3,941,679; E. W. Funk et al., U.S. Pat. No. 4,347,118; D. 0. Hanson, U.S. Pat. Nos. 4,139,450 and 4,071,433; H. E. Alford, et al., U.S. Pat. No. 4,067,796; H. W. Gagon, U.S. Pat. No. 4,342,639; and J. A. Gearhart, U.S. Pat. No. 4,315,815, as well as the references cited during prosecution and those referenced referred to in developing the background of the invention in each patent.
In general these patents describe techniques where sand is contacted in a series of extraction tanks and columns, with or without agitation, or where the sand is placed in a perforated container or a conveyor belt and the solvent is sprayed on the top and allowed to percolate through the bed of sand. In most cases, these techniques are designed to increase the extraction efficiency of the solvents being used. The other aspect most often mentioned are techniques to remove the solvent from the sand after the extraction stage; e.g., water displacement of the solvent from the extracted sand, multifluid bed driers, etc. Sands are conveyed between the various stages of these processes by accepted commercial practices; i.e., screw, slurry pumps, conveyor belts, etc.
Hastings (U.S. Pat. No. 4,311,561) teaches a countercurrent multistage vessel process. The last vessel in the series is filled with hot water as a means of removing entrained solvent from the sand prior to disposal.
Hart, et al. (U.S. Pat. Nos. 4,054,506 and 4,054,405) teaches a method of using ultrasonics to enhance the recovery of bitumen from tar sands.
Murray, et al. (U.S. Pat. No. 4,120,775) teaches a tar sand extractor design in which the leached tar sand is classified into fine and course fractions. The fine sand stays with the miscella, while the course fraction falls to the bottom where it is collected for removal from the extractor (fine sand retention permits easier washing and draining). In a second patent (U.S. Pat. No. 4,176,465), they teach a method for drying sand in a device designed to utilize the latent heat of vaporization of solvent vapors of the condensing solvent to preheat the sand entering the drier.
Pittman, et al. (U.S. Pat. No. 3,856,474) teaches an apparatus for extracting bitumen from tar sands by spraying solvent on tar sand conveyed on a perforated moving belt. Primary emphasis is on the design of the conveyor belt. In U.S. Pat. No. 4,029,568, they teach the use of high-pressure sprays, from 1-100 psi, with their conveyor belt extraction system. Their preferred solvents are methyl chloroform, trichloroethylene and perchloroethylene, because of "their high solvent effect, low boiling point, low specific heat and low heat of vaporization".
Karnofsky (U.S. Pat. No. 4,239,617) teaches a process to recover oil from diatomaceous earth through contacting the ore with a hydrocarbon solvent in a series of countercurrent extraction stages. The solvent is removed from the spent diatomite by first contacting it with water and secondly with steam. The oil-solvent solution is evaporated in multiple-effect evaporators followed by steam stripping.
Smith, et al. (U.S. Pat. No. 3,941,679) teaches a method using trichlorofluoromethane for the in situ and surface extraction of tar sands.
Funk, et al. (U.S. Pat. No. 4,347,118) teaches a process using C.sub.5 to C.sub.6 hydrocarbons. A two-stage process where a concentrated bitumen-solvent solution is separated in a classifier as an overflow and the course sand underflow is sent to a countercurrent extraction column for further extraction before entering a series of fluid bed driers. The patent emphasizes the use of multistaged fluid bed drying for complete removal of the solvent.
Hanson, et al. (U.S. Pat. No. 4,139,450) teaches a countercurrent extraction method for wet sands where the water is removed with hot solvent vapors prior to the extraction process. In U.S. Pat. No. 4,071,433, they use a technique where tar sand is slurried with oil, the course sand separated by centrifuge and the fine sand, oil, bitumen stream is fed directly to a coker.
Alford, et al. (U.S. Pat. No. 4,067,796) teaches a process involving a conditioning step with an alkaline aqueous solution followed by the extraction and separation of the tar sand with a hydrocarbon solvent, in a vessel which also contains water, thus forming two immiscible liquid phases for ease of sand separation.
Gagon (U.S. Pat. No. 4,342,639) teaches the extraction of tar sand with a halogenated solvent wherein the extracted sand is separated from the bitumen solvent solution by feeding the oil-solvent-sand slurry onto a conveyor system partially submerged in water. A halogenated solvent is important, because the oil-solvent solution must be heavier than water in order to affect separation.
Gearhart (U.S. Pat. No. 4,315,815) teaches a method of separating a solvent from bitumen by pressure reduction at elevated temperatures followed by steam stripping. A device to accomplish this is also described.
None of the above patents address the need to insure the complete removal of the solvent from the extracted bitumen prior to further refining. This, of course, is not a major need when nonhalogenated solvents are used as the extracting solvent, as most of the above patents so specify. However, even those who specify a halogenated solvent, e.g., Pittman, et al., Smith, et al., and Gagon, essentially ignore the solvent-bitumen separation problem. They specify technology such as flash distillation, a conventional evaporator and ambient temperature evaporation (thought to be applicable for summer desert environments) for solvent-bitumen separation. The concern for residual chlorides in crude oil or bitumen feeds to a refinery is universal throughout the petroleum industry. Past experience with chloride-contaminated crude oil refinery feed has been extremely negative; e.g., causing major corrosion damage to various refinery units as well as causing process upsets due to catalyst poisoning. Consequently, the use of chlorinated solvents for either bitumen or crude oil extraction is generally not considerable feasible.
It is difficult to remove solvents, even the low boiling methylene chloride, to contents much below 100 ppm by conventional techniques. With hydrocarbon solvents such low levels are acceptable because they are recoverable in the bitumen refinering process. However, it is not acceptable to have halogenated hydrocarbon contents in bitumen over 100 ppm and preferably not over 10 ppm, because the chloride is corrosive to refinery equipment and can harm catalysts used in the refining process. Therefore, a procedure is needed to reduce the chlorinated hydrocarbon content in the extracted bitumen to less than 10 ppm.